Gettiing to 32%

[billt4sf]

At the LDS I usually use, (which I like), they often prepare the tanks near the time I arrive. Sometimes when I analyze the tank it reads way less than 32% -- like around 28% or so. The explanation is "we just prepared them and the gas takes a while to become uniform. Don't worry -- it will be 32% before you use it". I think maybe they have a used a tank for air, then put Nitrox in it for me. Is this a safety issue? If I read 28% on some of the gas, couldn't it mean that part of the tank would be higher than 32%?

The tanks have an "O2 clean" sticker but not a large green "Nitrox" label. These are 100 cf steel tanks.

Thanks,

Bill

 

[kelemvor]

Gas does take some time to mix when blended. When I pick up my blends, they usually read high and then I retest when I dive and they read lower. One thing you can do is put the tank on the ground (might have to remove the boot) and roll it around for a few minutes. It further mixes the 02 and air in the bottle. This is something my blender showed me, and it definitely changes the reading you get on a freshly blended tank. Ideally you should get your own analyzer and retest before you get on the boat.

Maybe you could drop your tanks off a day earlier so they can sit for a little while after they're mixed?

In any event, the gas will mix eventually and you'll have some consistent value. Maybe calculate MOD based on 32 and nitrogen based on 28?

 

[RJP]

Yeah, that's why my shop has one of those paint-can-shaker things by the fill station.

[shakehead]

Do you analyze the tank again before you dive, or just when you pick them up at the shop? If that's the case - and what the shop guy is telling you is right - then you really have no idea what mix you're diving when you hit the water, do you?

For an EANx 32 partial-pressure fill the tank should read very close to 32 immediately upon filling - maybe 31.5? 32.5? The main reason for a freshly partial-pressure filled tank reading low is that the last gas in is air - and that gas can linger in the valve. If you purge the tank a bit prior to analyzing it the tank should read 32% - assuming that's what's in the tank - without any need to roll the tank around. The gas in the tank is pretty well mixed.

For example, if you're filling an empty 100cf HP steel tank to 32% you would put 457psi of O2 in the tank, and then add 2985psi of air to get to 32%. The dynamic forces of jamming the additional 2984psi of air into the tank will be more than sufficient to adequately mix the molecules in the tank.

I guess, technically, if you start with a tank that is already very full and needed to add only a bit of 02 and air you could have insufficient blending immediately after filling. But even that seems unlikely in practice. Or maybe if you do a fast, hot fill?

I've done partial-pressure fills of my own tanks hundreds of times and I have never had a tank analyze differently before hitting the water than it analyzed at the fill station.

 

[boulderjohn]

So are you able to retest them later to find out if it is true?

As someone who has blended a lot of nitrox, I agree that the blend can change in time, but in my experience it rarely changes more than 1%, and it does not take a real long time to get there.

The tanks do not have to have the big green nitrox labels on them. In my experience, those labels are becoming more and more scarce.

As far as your concern that the shop may be taking a used air tank and putting nitrox in it... If they are blending in the tank, the normal procedure is to put oxygen in and then top it off with air. If they are blending in the tank, they do not add premixed nitrox. If they have an oxygen booster, an expensive piece of equipment, they can add oxygen to a tank with air in it, but if they don't have that equipment, they really can't add oxygen to a tank with a decent amount of air in it, and they will normally bleed the tank down and start from scratch. Even if they do have a tank with a lesser amount of nitrox in it (including air, which is nitrox 21) and try to bring it up to a target level by adding oxygen, there is nothing wrong with that, as long as they hit the target.

To put it simply, there is no short cut that is going to benefit them other than the cost savings of the oxygen they may short you. the only issue is whether or not they hit the target. If I asked for 32%, I would not walk out of the shop with 28%.

 

[tomfcrist]

Could have to do with the mix getting richer as the tank cools as well......

 

[RJP]

Could have to do with the mix getting richer as the tank cools as well......

Hmm. This should be good...

 

[Jim Lapenta]

If you request 32 you should get a mix between 31 & 33 at the most if the blender knows what they are doing and accounting for a slight variation in analyzers. But even at that it's usually less. Like maybe 1/2% or so. If they are telling you that 28 is going to magically become 32 by the time you get to dive sight they are full of crap. Stop wasting your money and just dive air. They don't know what they are doing, are ripping you off, have a bad analyzer, or are just full of it. I would not get mix from them of any kind.

Sent from my DROID X2 using Tapatalk 2

 

[dumpsterDiver]

I've seen some pretty big changes in mix, before and after rolling the tanks. Rolling the freshly filled tanks will mix up the nitrox.

 

[LowDrag]

I am not nearly as experienced as the others that have posted on this subject but I have never heard of 32% mix starting out at 28% and ending up at 32% later. That's a new one to me and a bit suspicious IMO. When my LDS fills 32% or 36% and we analyze the tank it is as stated above +/- ~1%.

 

[abyss_scuba]

I did a mix recently for someone wanting a Deco mix of 70%. When I first analyzed the tank it read somewhere around 53% after rolling around on the floor with no additional gas added it jumped to somewhere around 67.9. So those saying you are getting ripped off are going off half cocked. I would bring your tanks a day or two in advance and I bet your tanks should be much closer to your desired mix. Then go back with time to spare if the mix is off have them redo it. People are always in such a rush to write off scuba shops without knowing (or wanting to know) all the details. Get back to us and let us know what the mix is after the tanks have had time to settle please.